In recent years, in order to increase the recording density of magnetic disk memories, a magnetic head has had an extremely small float thickness of 10 nm or smaller. In the production process of a magnetic disk substrate, a surface polishing process is essential, and the surface polishing is carried out with, for example, an abrasive containing colloidal silica.
The abrasive is required to provide good surface smoothness (for example, surface roughness [Ra] and waviness [wa]) and not to cause surface defects such as scratches and pits.
Also in the semiconductor field, wirings have been made finer in highly integrated circuits and faster operating frequencies. In the production process of semiconductor devices, a pattern formation surface is also required to have further smoothness.
In a planarization process of these magnetic disk substrates and semiconductor substrates, subsequent to the polishing process with an abrasive containing colloidal silica, the colloidal silica as abrasive grains and fine particles are removed by washing.
For the washing, an aqueous solution of an acidic or alkaline chemical agent is used. Usable examples of the acidic chemical agent include a compound containing a fluorine ion, such as hydrofluoric acid, ammonium fluoride, ammonium hydrogen difluoride, and fluoroboric acid, sulfuric acid, nitric acid, hydrochloric acid, acetic acid, citric acid, malic acid, oxalic acid, and perchloric acid. Usable examples of the alkaline chemical agent include sodium hydroxide, potassium hydroxide, calcium hydroxide, ammonia, and amines. An aqueous solution of such an acidic or alkaline chemical agent may contain a surfactant such as a sodium alkylbenzene sulfonate, a polyoxyethylene alkyl ether sulfate, and a dioctyl sulfosuccinate; and a chelating agent such as sodium tripolyphosphate, sodium pyrophosphate, zeolite, and sodium ethylenediaminetetraacetate.
The colloidal silica used as the abrasive has a spherical shape or substantially a spherical shape, and thus can be removed by a related-art washing method. However, recently, it has been revealed that there are particles that cannot be easily removed by the related-art washing method.
As a method of obtaining an alkali silicate aqueous solution that is substantially free from particles having a particle size of 1 nm or more, a method in which an alkali silicate aqueous solution is previously adjusted to have a viscosity of 1 to 50 mPa·s and is passed through an ultrafiltration membrane having a molecular weight cut-off of 15,000 or less is disclosed (Patent Document 1).